JP2000-299070(A) discloses a structure of a magnetron capable of suppressing a spurious level low. In this disclosure, as shown in FIG. 2, a configuration of a pair of the pole pieces 5 which gives a magnetic field to an acting space 3 between a cathode 2 and the anode vanes 4 is shown in FIG. 1 of the disclosure. The opposing faces of the pole pieces 5 have a truncated cone shape having a taper of 20 degrees or greater. Because the opposing faces of the pole pieces 5 are formed in the truncated cone shape, the magnetic flux density in the acting space typically becomes higher at the both ends than the central position in the axial direction (refer to a magnetic flux density distribution of FIG. 3 of this disclosure).
Because the magnetic flux density of the acting space is higher toward the both ends in the axial direction, a spurious level of an electric wave transmitted from the magnetron is suppressed low. Generally, since an electric field is disturbed in the boundary planes at the both ends of the cathode 2 and the anode vane 4 in the axial direction, the magnetic field may act to reduce the electronic distribution near the boundary planes.
Therefore, adopting the shape disclosed in JP2000-299070(A), the magnetic field near the boundary is strengthened to bounce electrons toward the weaker side of the magnetic field. This is a phenomenon in which an area called the “mirror” where lines of magnetic force are converged in a funnel shape receives repulsion and, thus it is referred to as “the mirror effect.” With this effect, the distribution of electrons is brought toward the central part of the cathode 2 in the axial direction to increase a rate of electrons uniformly moving. As a result, unnecessary radiation (spurious emission) is reduced.
However, with the configuration of JP2000-299070(A), as shown in FIG. 3 of this disclosure, the magnetic flux density in the acting space would be a distribution such that it entirely inclines linearly in the radial direction. Therefore, the suppression of the spurious emission is limited due to such non-uniformity of the magnetic flux density distribution over the entire acting space area in the radial direction. Particularly, magnetrons for radar are typically used to generate pulsed microwaves at a predetermined repetition frequency. For this reason, it may be necessary to consider the suppression of the spurious emission generated especially during the rising period of a pulse period during which the microwaves occur. Apparently, JP2000-299070(A) does not consider this point at all.
Recently, while microwave-applied instruments and the applications thereof have increased, the spurious regulations are becoming strict more and more. Meanwhile, with a configuration in which a filter intervenes, the suppression of the spurious emission which differs only somewhat in frequency comparing with the fundamental wave is limited. In addition, because the configuration does not conform to the recent demands for size reduction of the instruments, the suppression of the spurious emission is now demanded simply for modifications of the magnetron per se.